/*
Copyright 2018 The Kubernetes Authors.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

package queue

import (
	"container/heap"
)

// PriorityQueue implements a scheduling queue.
type PriorityQueue struct {
	queue priorityQueue
}

// LessFunc is the func declaration used by sort or priority queue.
type LessFunc func(interface{}, interface{}) bool

// CompareFn is the func declaration used by sort or priority queue.
type CompareFn func(interface{}, interface{}) int

type priorityQueue struct {
	items  []interface{}
	lessFn LessFunc
}

// NewPriorityQueue returns a PriorityQueue
func NewPriorityQueue(lessFn LessFunc) *PriorityQueue {
	return &PriorityQueue{
		queue: priorityQueue{
			items:  make([]interface{}, 0),
			lessFn: lessFn,
		},
	}
}

// Push pushes element in the priority queueName
func (q *PriorityQueue) Push(it interface{}) {
	heap.Push(&q.queue, it)
}

// Pop pops element in the priority queueName
func (q *PriorityQueue) Pop() interface{} {
	if q.Len() == 0 {
		return nil
	}

	return heap.Pop(&q.queue)
}

// Empty check if queue is empty
func (q *PriorityQueue) Empty() bool {
	return q.queue.Len() == 0
}

// Len returns Len of the priority queue
func (q *PriorityQueue) Len() int {
	return q.queue.Len()
}

func (pq *priorityQueue) Len() int { return len(pq.items) }

func (pq *priorityQueue) Less(i, j int) bool {
	if pq.lessFn == nil {
		return i < j
	}

	// We want Pop to give us the highest, not lowest, priority so we use greater than here.
	return pq.lessFn(pq.items[i], pq.items[j])
}

func (pq priorityQueue) Swap(i, j int) {
	pq.items[i], pq.items[j] = pq.items[j], pq.items[i]
}

func (pq *priorityQueue) Push(x interface{}) {
	(*pq).items = append((*pq).items, x)
}

func (pq *priorityQueue) Pop() interface{} {
	old := (*pq).items
	n := len(old)
	item := old[n-1]
	(*pq).items = old[0 : n-1]
	return item
}
